1. Field of the Invention
The present invention relates to a radio communication apparatus used for signal transmission and reception in a mobile radio communication system which is used in a portable mobile radio telephone system, a cordless telephone system and so forth. More particularly, this invention relates to a gain control circuit provided in a radio communication apparatus which employs a digital communication system.
2. Description of the Related Art
Recently, a "dual-mode type" communication system, which employs a digital communication system that provides better tap-free environment and can ensure more effective utilization of the radio frequency than an analog communication system, becomes gradually popular as an example of the application of a mobile radio communication system. The "dual-mode" system is the communication system which selectively uses analog and digital modes to execute radio communication. In analog mode, carrier signals are, for example, frequency-modulated with analog speech signals in a transmitter. In digital mode speech signals are encoded and carrier signals are modulated with the coded speech signals by a digital modulation system, such as .pi./4 shifted DQPSK (Differentially encoded Quadrature Phase Shift Keying), before transmission.
In general, a radio communication apparatus which is used in a communication system of the above digital communication system, transmits information on a waveform that has a predetermined phase and a predetermined amplitude. Once a received signal becomes saturated in the receiver system, therefore, this radio communication apparatus cannot perform accurate demodulation of the modulated waveform thereafter. Thus, this type of radio communication apparatus is generally provided with an automatic gain control (AGC) circuit in the intermediate frequency stage in the receiver system to prevent the saturation of the received signal.
FIG. 6 exemplifies the basic structure of a conventional AGC circuit 30. In this diagram, the unnecessary frequency component of a received intermediate-frequency signal, amplified by a variable-gain type intermediate frequency amplifier 31, is eliminated by an intermediate-frequency-pass filter 32. The resulting signal is input to a detector 34 as well as to a digital demodulator (not shown). This detector 34 detects the signal level of that received intermediate-frequency signal and sends the detected value to an error amplifier 35. This error amplifier 35 detects the differential voltage between the detected value and a reference voltage generated by a reference voltage generator 36. This differential voltage is then negative feedbacked as a gain control signal via a gain control terminal of the intermediate frequency amplifier 31 to change the gain of this amplifier 31. The gain of the intermediate frequency amplifier 31 is controlled so as to make the differential voltage to zero. As a result, the amplitude of the received intermediate-frequency signal is controlled to be always kept at the value corresponding to the reference voltage.
In actual application, however, the radio communication apparatus having the above conventional AGC circuit 30 has the following shortcoming. A mobile radio unit may simultaneously receive both the radio wave on a radio channel that is currently used in communication and the radio wave on an adjacent channel which has a greater reception electric field intensity than the former radio wave, thus causing crosstalk or radio interference. The former radio wave will be hereinafter referred to as "desired wave" (i.e., the desirable frequency for communication), and the latter as "interference wave." In such crosstalk, since the passband of the intermediate-frequency-pass filter 32 is generally set so wide as to permit the received intermediate-frequency signal on the adjacent channel to pass, the received intermediate-frequency signal of the interference wave will not be filtered sufficiently and will thus be output at a higher level than the level of the received intermediate-frequency signal of the intermediate frequency amplifier from the intermediate-frequency-pass filter 32. Accordingly, the detector 34 detects the received signal level of the interference wave on the adjacent channel, not the received signal level of the desired wave. As a result, the error amplifier 35 in the next stage outputs a gain control signal corresponding to the received signal level of the interference wave. This causes the intermediate frequency amplifier 31 to perform an erroneous gain control. The erroneous function in a channel selectivity operation, which has originated from the AGC circuit 30 with the above-described conventional structure, deteriorates the characteristic of the selectivity among adjacent channels, and will thus raise a very undesirable problem in the application of the radio communication apparatus.
As an improvement to overcome this problem, the passband of the intermediate-frequency-pass filter may be set to such a "narrow-band" that does not cause crosstalk. This setting however raises another problem. That is, the "group delay ripple component" in the characteristic of a modulated wave becomes larger, so that, particularly, the "bit error rate (BER)" in a digital demodulated signal is deteriorated. The "group delay ripple" is the width of fluctuation of group delay in the passband. This width of fluctuation depends on the length of the group delay, and the smaller the amplitude is, the more accurate the demodulation of the modulated wave can be. In dual mode, particularly, in digital mode, as the band of the filter in use is set narrower, the group delay ripple varies greatly. This increases the group delay ripple so that the accurate demodulation is no longer possible.